Joanna Sakowska

Postgraduate Research Student

Academic and research departments

Astrophysics Research Group.

My research project

My publications


We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (CenI) and two Milky Way (MW) $\delta$ Scuti/SX Phoenicis stars based on multi-epoch $giz$ DECam observations. The two RRc stars are located within 2 times the half-light radius (r$_h$) of Cen I, while the RRab star (CenI-V3) is at $\sim6$ r$_h$. The presence of three distant RRL stars clustered this tightly in space represents a 4.7$\sigma$ excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of $\mu_0 = 20.354 \pm 0.002$ mag ($\sigma=0.03$ mag), a heliocentric distance of D$_\odot = 117.7 \pm 0.1$ kpc ($\sigma=1.6$ kpc), with systematic errors of $0.07$ mag and $4$ kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars ($f_{cd}$) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of $f_{cd} = 0.28$. While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs ($f_{cd}$ $\sim$ 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies.

We present the discovery of a candidate ultra-faint Milky Way satellite, Eridanus IV (DELVE J0505$-$0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). Eridanus IV is a faint ($M_V = -4.7 \pm 0.2$), extended ($r_{1/2} = 75^{+16}_{-13}$ pc), and elliptical ($\epsilon = 0.54 \pm 0.1$) system at a heliocentric distance of $76.7^{+4.0}_{-6.1}$ kpc, with a stellar population that is well-described by an old, metal-poor isochrone (age of $\tau \sim 13.0$ Gyr and metallicity of ${\rm [Fe/H] \lesssim -2.1}$ dex). These properties are consistent with the known population of ultra-faint Milky Way satellite galaxies. Eridanus IV is also prominently detected using proper motion measurements from Gaia Early Data Release 3, with a systemic proper motion of $(\mu_{\alpha} \cos \delta, \mu_{\delta}) = (+0.25 \pm 0.06, -0.10 \pm 0.05)$ mas yr$^{-1}$ measured from its horizontal branch and red giant branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.

A Drlica-Wagner, Jeffrey L Carlin, David L Nidever, P. S. Ferguson, N Kuropatkin, M. Adamów, W. Cerny, Yumi Choi, J Esteves, C. E. Martínez-Vázquez, S Mau, A. E Miller, B. Mutlu-Pakdil, E Neilsen, K Olsen, A.B Pace, A. H. Riley, Joanna D Sakowska, DJ Sand, L. Santana-Silva, EJ Tollerud, D Tucker, AK Vivas, E Zaborowski, A. Zenteno, T Abbott, S Allam, K Bechtol, C Bell, Eric F. Bell, P Bilaji, C. R Bom, Julio A. Carballo-Bello, D. Crnojević, Maria-Rosa L. Cioni, A Diaz-Ocampo, T. L de Boer, DENIS ERKAL, RA Gruendl, D. Hernandez-Lang, ASHLEY KATE HUGHES, DEBORAH JAMES, L Johnson, TIANRU LI, Y.-Y. Mao, D Martinez-Delgado, POL MASSANA ZAPATA, M. McNanna, R Morgan, E. O. Nadler, NED Noël, A. Palmese, AHG Peter, ES Rykoff, JLG Sanchez, N. Shipp, Joshua D. Simon, A Smercina, M Soares-Santos, G Stringfellow, K. Tavangar, Roeland van der Marel, Alistair Walker, RH Wechsler, J. Wu, B Yanny, MF Fitzpatrick, Luan Huang, A Jacques, R Nikutta, AMY SCOTT (2021)The DECam Local Volume Exploration Survey: Overview and First Data Release, In: The Astrophysical journal. Supplement series256(1) IOP Publishing

The DECam Local Volume Exploration survey (DELVE) is a 126-night survey program on the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. DELVE seeks to understand the characteristics of faint satellite galaxies and other resolved stellar substructures over a range of environments in the Local Volume. DELVE will combine new DECam observations with archival DECam data to cover ~15,000 deg2 of high Galactic latitude (|b| > 10°) southern sky to a 5σ depth of g, r, i, z ~ 23.5 mag. In addition, DELVE will cover a region of ~2200 deg2 around the Magellanic Clouds to a depth of g, r, i ~ 24.5 mag and an area of ~135 deg2 around four Magellanic analogs to a depth of g, i ~ 25.5 mag. Here, we present an overview of the DELVE program and progress to date. Furthermore, we also summarize the first DELVE public data release (DELVE DR1), which provides point-source and automatic aperture photometry for ~520 million astronomical sources covering ~5000 deg2 of the southern sky to a 5σ point-source depth of g = 24.3 mag, r = 23.9 mag, i = 23.3 mag, and z = 22.8 mag. DELVE DR1 is publicly available via the NOIRLab Astro Data Lab science platform.

David L Nidever, Knut Olsen, Yumi Choi, Tomas Ruiz-Lara, Amy E Miller, L. Clifton Johnson, Cameron P. M Bell, Robert D Blum, Maria-Rosa L Cioni, Carme Gallart, Steven R Majewski, Nicolas F Martin, Pol Massana, Antonela Monachesi, Noelia E. D Noel, Joanna D Sakowska, Roeland P van der Marel, Alistair R Walker, Dennis Zaritsky, Eric F Bell, Blair C Conn, Thomas J. L de Boer, Robert A Gruendl, Matteo Monelli, Ricardo R Munoz, Abhijit Saha, A. Katherina Vivas, Edouard Bernard, Gurtina Besla, Julio A Carballo-Bello, Antonio Dorta, David Martinez-Delgado, Alex Goater, Vadim Rusakov, Guy S Stringfellow (2020)The Second Data Release of the Survey of the MAgellanic Stellar History (SMASH), In: The Astronomical journal161(2)74 IOP

The Large and Small Magellanic Clouds (LMC and SMC) are the largest satellite galaxies of the Milky Way and close enough to allow for a detailed exploration of their structure and formation history. The Survey of the MAgellanic Stellar History (SMASH) is a community Dark Energy Camera (DECam) survey of the Magellanic Clouds using ~50 nights to sample over ~2400 deg2 centered on the Clouds at ~20% filling factor (but with contiguous coverage in the central regions) and to depths of ~24th mag in ugriz. The primary goals of SMASH are to map out the extended stellar peripheries of the Clouds and uncover their complicated interaction and accretion history as well as to derive spatially resolved star formation histories of the central regions and create a "movie" of their past star formation. Here we announce the second SMASH public data release (DR2), which contains all 197 fully calibrated DECam fields including the main body fields in the central regions. The DR2 data are available through the Astro Data Lab hosted by the NSF's National Optical-Infrared Astronomy Research Laboratory. We highlight three science cases that make use of the SMASH DR2 data and will be published in the future: (1) preliminary star formation histories of the LMC, (2) the search for Magellanic star clusters using citizen scientists, and, (3) photometric metallicities of Magellanic Cloud stars using the DECam u-band.